1. Field of the Invention
This invention relates to an electric motor comprising an armature and a field magnet which are movable relative to each other.
2. Related Art
Recently, a variety of electric motors have been developed. One example of conventional electric motors is a three-phase motor as shown in FIGS. 27 and 28. The motor is of a so-called "2-3 (two magnetic poles--three core poles) structure". That is, a hollow-cylinder-shaped field magnet 2 is fixedly mounted on the inner surface of a casing 1, and an armature 3 is rotatably supported, for instance, inside the field magnet 2. The field magnet 2 is so magnetized that two different magnetic poles N and S are formed along the circumference. The armature 3 includes three protruded poles 3a, 3a and 3a which are set near the inner surface of the field magnet 2, to collect the magnetic flux. Coils 3b, 3b and 3b are wound on those three protruded poles 3a, 3a and 3a, respectively.
Another example of the conventional electric motors is a multipolar electric motor, which is as shown in FIG. 29. In the motor, a hollow-cylinder-shaped field magnet 12 fixedly fitted on the inner surface of a casing 11 is so magnetized that a number of different magnetic poles N and S are formed along the circumference at predetermined intervals, and an armature 13 is rotatably supported, for instance, inside the field magnet 12. The armature 13 includes a number of protruded poles 13a, 13a, . . . , on which coils 13b, . . . are wound respectively.
Those conventional electric motors suffer from a difficulty in that they are insufficient in the so-called "T/N characteristic value". The T/N characteristic value is the quotient obtained by dividing the torque T by the number of revolutions N in the motor's T-N characteristic. More specifically, ##EQU1## where T.sub.S : Start torque
N.sub.O : No-load revolution number PA1 K.sub.E : Counter electromotive force constant PA1 K.sub.T : Torque constant PA1 R: Internal resistance
The T/N characteristic value represents the magnitude of the T-N characteristic which is one of the basis characteristics of the motor. Hence, it can be utilized for comparison of the power of a motor. For instance, an electric motor of T/N=3 is able to provide the same T-N characteristic as three electric motors of T/N=2 which are operated at the same time. More specifically, the T/N characteristic value is proportional to about the square (more precisely the 5/3-rd power) of the volume of the motor, and substantially proportional to BH.sub.MAX of the field magnet. Hence, to use a larger motor or a stronger magnet is to increase the T/N characteristic value.
When the T/N characteristic value is increased, then the following can be achieved:
1) Increasing the generated torque
2) Decreasing the rising time
3) Increasing the torque constant and the counter electromotive force constant
4) Decreasing the rated current
5) Decreasing the loss (copper loss)
6) Improving the efficiency
7) Decreasing the generated heat
8) Increasing the output
9) Decreasing the effect of the variation in load on the variation in the number of revolutions
In addition, depending on the T/N characteristic value, the following are permitted:
1) Reducing the size, thickness and weight of the motor
2) Reducing the manufacturing cost, for instance, by looking over the material cost
3) Increasing the degree of freedom in designing the motor.
As is apparent from the above description, most of the variety of propositions made for electric motors are to increase the T/N characteristic value. That is, as for an electric motor, the demands for reducing the size, decreasing the power consumption, saving the material, and reducing the manufacturing cost are based on (T/N characteristic value)/(size), and (T/N characteristic value)/(cost). Thus, heretofore, how to efficiently obtain the T/N characteristic value is a problem to be solved. In this connection, what are important are for instance as follows:
1) Decreasing the size, reducing the thickness, and decreasing the weight
2) Increasing the starting torque
3) Decreasing the rising time
4) Decreasing the current value
5) Reducing loss (copper loss)
6) Rationalizing
Thus, most of the propositions made for the technique of electric motor are studies on the improvement of the T/N characteristic value. In practice, in the studies, the differences in T/N characteristic value are in a range of from 5% to 10%.
The T/N characteristic value is determined by factors P (the number of magnetic poles), .PHI. (effective magnetic flux), H (the number of parallel coils), A (coil sectional area), and L (coil length per T); that is, EQU T/N characteristic value.varies.P.sup.2 .PHI..sup.2 H A/L (2)
Hence, by suitably selecting those factors, the T/N characteristic value can be maximized. In this case, it is a key point how to increase P.sup.2 .times..PHI..sup.2.
In view of the foregoing, the electric motor of the so-called "2-3 structure" described with reference to FIGS. 27 and 28 may be considered as follows: In the three-phase motor, the number of magnetic poles and the number of protruded poles are minimum, and, when the armature is positioned as shown in FIG. 27, the total magnetic flux of the N pole concentrates at one protruded pole 3a as indicated by the arrows. Therefore, the effective magnetic flux .PHI. is large; however, the improvement of the T/N characteristic value is limited because the number of magnetic poles (P) is two (2).
On the other hand, in the multipolar electric motor shown in FIG. 29, the number of magnetic poles (P) and the number of parallel coils (H) are larger, and the coil length (L) is shorter, so that the T/N characteristic value is improved as much. However, the confronting area of each protruded pole 13a of the armature 13 through which the former 13a confronts with the field magnet 12 is smaller, and the magnetic flux is dispersed; that is, one and the same magnetic flux is split for all the poles. Hence, although the number of magnetic poles (P) is larger, the effective magnetic flux (.PHI.) is smaller. Thus, the value of P.sub.2 .times..PHI..sup.2 is not changed. On the other hand, the number of parallel coils (H) can be increased; however, this increase is canceled by the decrease in the coil sectional area (A). Hence, although the motor is intricate in structure, the T/N characteristic value cannot be increased so much. Accordingly, in the case of the multi-polar electric motor, a magnet high in BH.sub.MAX is employed to increase the effective magnetic flux (.PHI.) thereby to improve the T/N characteristic value.
As is apparent from the above description, the conventional electric motor suffers from a difficulty in that the T/N characteristic value cannot be increased without employment of the strong magnet, which results in an increase in the manufacturing cost.